Cosmological Redshift

I would like to hear some interpretations of this phenomenon, especially in regards to what happens to the energy of photons in cosmological redshift – where do YOU think it goes? Is it lost or is it conserved?

Originally posted by (Q) I would like to hear some interpretations
of this phenomenon, especially in regards
to what happens to the energy of photons
in cosmological redshift – where do YOU
think it goes? Is it lost or is it conserved?

Space-time expands and the EM waves do too.
I'm not certain about how the energy is
treated, I suppose you could say nothing
dissappears and it's just like normal
space-time curves or there are those theories
dealing with quentessense or something like
that and say that it "fuels" the expansion.

Originally posted by (Q) I would like to hear some interpretations of this phenomenon, especially in regards to what happens to the energy of photons in cosmological redshift – where do YOU think it goes? Is it lost or is it conserved?

this question is one reason why physicforums is the greatest thing since sliced bread. this is a great question

i want to hear what some gurus say about this (tom, damgo, etc)

note that your questions assumes that it goes somewhere
which means you are assuming a conservation law
but there is no energy conservation law in GR, I think.
or any momentum conservation law either.

you are assuming that if energy is lost (from the light being stretched out and getting a longer wavelength) that it must
GO somewhere----because in local SR physics energy is conserved.

but expansion of space is a GR thing----so you must ask if
there is a conservation law in that context. Maybe there is!!!
I am no expert. But I do not know of one, for whatever that is worth.

I heard a neat thing about GR. An amoeba can move in curved space merely by changing shape.

this seems to contradict conservation of momentum

he does not need any rockets to push him. he just changes is shape a couple of times and behold! he is in a different place.

this was at an MIT archive of recent online physics papers or something, they thought it was cool. It is good peer-reviewed physics even tho it sounds wacky.

so if anybody thinks he knows of a conservation law for momentum in GR he will have to explain this traveling amoeba

(Q) you are great for asking this question. maybe it will get a good answer from the likes of damgo

Originally posted by (Q) I would like to hear some interpretations of this phenomenon, especially in regards to what happens to the energy of photons in cosmological redshift – where do YOU think it goes? Is it lost or is it conserved?

I think that Redshift is a change in the wavelength of the light reaching us, not a change in the energy of any particular photon(s) at all.

Remember, the Alien looking from the "other side" sees the same light source blue-shifted, same light source = same photon energy. The photon energy and the perceived wavelength are two different animals.

But doesn't the relation between wavelength and energy for a photon imply that when a photon is Doppler shifted (from our point of view) that the amount of energy we measure for it has changed? So the photons reaching the alien actually have more momentum (if absorbed) than photons absorbed over here.

Is an "apparent" change in the energy of a photon different than an actual one (what is an "actual" one, anyway)?

Oh that... well... it's not entirely accurate. It's a study of how amoebas move in water that gave a theoretical way of cheating newton's 3rd law - if space is curved, you can exploit the gravitational gradient by generating motion through moving the craft's centre of gravity. It's sort of akin to tacking a yacht in a headwind....

Originally posted by LURCH Not so. An alien on the other side would see the light red-shifted, just as we do. If he is the same distance away, he'll see the red-shift about the same amount, too.

Whoa? Let's say we are here and the alien is way over there, and moving about equally relative to us. Place a fast-moving light source smack in the middle between us and the alien. The light source moves quickly away from us and toward the alien. We see a "redshift" (wavelength change) because its moving away from us. The alien sees a blueshift because it is moving toward him.

Say a receding spaceship is flashing lights at us. In the spaceship frame the light is not redshifted and retains its energy.
In the earthbased frame looking at the spaceship flashing lights at us, the light was always of a lower energy lower frequency longer wavelength, so it retains its energy.

Everything is normal.

But this guy (Q) was talking about the COSMOLOGICAL redshift which happens from space stretching out while the light is under way towards us. this is a GR thing. It is not relative motion of source and receiver, but an effect of expansion of space itself.

He is saying 'what happened to all that energy?' I think IMHO that it just went away. What a huge amount of energy to have just ceased to exist! All the light from all the galaxies, for the whole history of the universe, most of which is still flying on its way somewhere, has been deprived of a large percentage of its energy!

Hey drag! I do not have the URL. There is an MIT archive of online physics goodies and I found it there. Maybe I will come across it again. Of course it was not a real amoeba, I just called it that. The journal article was about an object that can change its shape and, by using locally curved spacetime, can move around by changing shape thru a cycle that leaves it the same original shape but in a different place. If I see it again I'll post it.

Originally posted by marcus
Hey drag! I do not have the URL. There is an
MIT archive of online physics goodies and I
found it there. Maybe I will come across it
again. Of course it was not a real amoeba,
I just called it that. The journal article
was about an object that can change its
shape and, by using locally curved spacetime,
can move around by changing shape thru a
cycle that leaves it the same original
shape but in a different place. If I see
it again I'll post it.

Well it seems like an EXTREMELY "slow and
painfull" proccess, unless you can do
some near c movements of course but that's
a bit too much for an amoeba I guess.

But this guy (Q) was talking about the COSMOLOGICAL redshift which happens from space stretching out while the light is under way towards us. this is a GR thing. It is not relative motion of source and receiver, but an effect of expansion of space itself.

And also some things about all of the "missing energy".

Ok, I'll buy that in the context of "cosmological" redshift. But, even GR, Hawking, Wheeler, et al won't throw out thermodynamics; they require it. So, the energy takes an equivalent transformation to "something". Your (Marcus') description of the huge amount of energy available from this effect since expansion began might be "the" source of the "repulsive force" ("dark energy"). Just thinking out loud. This might seem to fit in with what Linde recently proposed, which was posted a day or two ago.(Expansion then contraction)

Originally posted by Labguy Whoa? Let's say we are here and the alien is way over there, and moving about equally relative to us.

But that's just it; he won't be (moving equally, that is). If things at great distances from one another were stationary relative to one another, there would be no cosmological redshift.* If we look at a light source one billion lightyears away, and an alien is also one billion lightyears away from that same light source (but on the "far side), he will see about the same redshift as we. If the alien were to look at us, and we at him, we would each measure the other's redshift to be about twice what it is for the intermediate object (the light source).

Staff: Mentor

Originally posted by Zefram But doesn't the relation between wavelength and energy for a photon imply that when a photon is Doppler shifted (from our point of view) that the amount of energy we measure for it has changed? So the photons reaching the alien actually have more momentum (if absorbed) than photons absorbed over here.

Is an "apparent" change in the energy of a photon different than an actual one (what is an "actual" one, anyway)?

Therein lies the problem - the photon doesn't change its wavelength. It is created at that wavelength because the source is moving. There is no missing energy.

Doppler shift works the same way for sound as well. With a moving source, the sound is created with a doppler shift.

Take a galaxy that we see receding, and its light red-shifted. People living in that galaxy do not see their light redshifte, it seems normal to them and it doesn't occur to them that any energy has been lost. What they do see red-shifted is our light. And maybe they ask where the energy went in our case.

Neither we nor the aliens are speeding, the distance beteen us is growing and it produces the effect of speed including red-shift. The stars in galaxy X are glowing with their usual spectra.

selfAdjoint and russ, if you maintain a certain
permanent rest frame in the Universe you will
see the expansion of space-time decrease the
energy of at least photons and possibly
also the kinetic energy of all rest-mass particles
(though the rest masses appear to stay the same).

the original post by (Q) called attention to the energy that has been lost from the universe by wavelength stretching caused by expansion of space.

this energy can be calculated---most simply in c=G=hbar=k=1 units.

Almost all the redshift energy loss since year 300,000 after BB has been from CMBR photons which were released around year 300,000 and have experienced redshift 1100.

This loss is overwhelmingly greater than that in light emitted by galaxies.

The temp of CMB is 2E-32
The energy density (energy per unit vol) of CMB is equal
to the fourth power of the temp, multiplied by &pi;2/15.
that factor is about 2/3
So the density comes out close to E-127
and multiplying by 1100 one gets about E-124.
It is a small energy density compared with dark energy
have to go will proofread later

Staff: Mentor

Originally posted by drag selfAdjoint and russ, if you maintain a certain
permanent rest frame in the Universe you will
see the expansion of space-time decrease the
energy of at least photons and possibly
also the kinetic energy of all rest-mass particles
(though the rest masses appear to stay the same).

Live long and prosper.

Aether? Unnecessary. Just like sound - but now both the observer and the source are moving. And it doesn't matter what frame of reference you use (Relativity) you still get the exact same doppler shift. The frequency of the light does not change while it is traveling unless acted upon by a gravitational field.